Drilling is an example of a manufacturing operation that requires tooling which inherently wears out or fails as the operation proceeds. When a small drill is used on a large machining center, a worn drill will always fail if the use thereof is continued beyond a particular wear limit. If the wear process occurred at a predictable rate a drill could be replaced at the optimum time in accordance with this rate. However, the wear process is not predictable with any degree of certainty and depends upon such variables as cutting edge sharpness, symmetry of the cutting edge, local change in material hardness, and lubrication. Thus, one drill may drill more than 2,000 holes during the lifetime thereof, while another may drill less than 100 holes before failure.
It is evident that a number of benefits would result from the ability to measure the wear of a cutting tool during the cutting process. For example, productivity would increase because optimum tool replacement times could be determined. Further, the ability to be able to determine that a tool has failed is useful since with this information the machining center can be prevented from attempting to feed the remains of a machine tool into the work. It will be appreciated that it is much easier to remove a broken drill if part of the drill remains above the surface of the workpiece. In this regard, "peck" drilling, the drill may be broken off flush with the workpiece surface or driven into the hole on the second peck after the drill has broken. Alternatively, the broken drill may slide off the piece of drill embedded in the workpiece and severely damage the surface.
Prior art systems that measure the power consumption of the machining center as an indication of tool wear are described in Beer, Larry D., "Power Consumption - A Measure of Tool Performance", Technical Paper MR79-398, Am. Soc. of Mechanical Engineers. Such systems can, however, only be used when a measurable amount of power is consumed in the machining operation in excess of that used for turning the spindle motor. Other methods such as measuring the cutting forces or temperature have generally been unsuccessful when a small tool is used in a large machining center.
Other prior art systems use signature analysis of the vibration from the machining operation as an indication of tool wear. Some systems of this type are described in Micheletti, G. F., Rossetto, S., and Ponti, M., "Tool Vibration Pattern and Tool Life on Automatic Screw Machine", Advances in M.T.D.R., Vol. A. Sept. 1970, 145-159; Young, F. W., "An Investigation of Available Signals for Adaptive Control Machine Tools", S.M. Thesis, Mechanical Engineering Dept., Mass. Inst. of Tech. May 1970; and Gaudreau, M., "An On-Line Technique for Tool Wear Measurement", M.S. Thesis, Dept. of Aeronautic and Astronautics, Mass. Inst. of Tech., June 1975. These systems have been of limited success. Other systems use spectral analysis to determine the amount of tool wear and this approach has disadvantages with respect to constant complexity. Such systems are described in Edwin, A., and Vlach, T., "A New Approach to Tool Wear Monitoring", Proceedings of the 27th Annual Conference and Exhibition of the Instrumentation Society of America, Oct. 1972; Weller, E. J., and Welchbrodt, B., "Listen to Your Tools-They're Talking to You", Technical Paper No. MR67444, Soc. of Mfg. Engineers; Weller, E. J. Schrier, H.M., and Welchbrodt, B., "What Sound Can be Expected from a Worn Tool?", A.S.M.E., J. Eng. for Industry, Aug. 1969, pp. 525-534; Lutz, J., Societe National des Petroles D'Aquitaire, Paris, France; and U.S. Pat. No. 3,714,822, entitled "Process Measuring Wear on a Drilling Tool".
Prior art patents of interest include U.S. Pat. Nos. 3,745,815 (Bentone et al.) and 3,979,739 (Bircchall). The Bentone et al patent discloses a device for evaluating the vibrations of a revolving member such as a bearing which device includes a detecting head incorporating an accelerometer, a detecting and amplifying circuit for amplifying and filtering signals generated by the detecting head in response to the vibrations referred to, a first amplitude discriminating circuit, a monostable multivibrator, an integrating circuit, a further amplitude discriminating circuit and relay. The monostable multivibrator produces pulses responsive to signals of an amplitude in excess of a threshold value set by the first discriminator circuit and the relay is activated only when a predetermined number of pulses (10 in a specific embodiment is received by the integrator. When this number of pulses is received, the resultant integrated output voltage is in excess of the threshold level set by the second amplitude discriminator and the relay is energized to, for example, automatically reject the bearing under test.
The Birchall patent discloses an apparatus for the detection of vibrations in rotating machinery wherein a transducer generates a signal whose amplitude corresponds to the amplitude of the machine under test and a comparator produces distinct outputs, i.e., a logic "1" or "0", depending on whether the transducer signal lies outside an acceptable range. If an "unacceptable" output is produced, the time during which this output persists is measured and the system is reset if the output does not persist for a predetermined time period. If the output does persist for that period, an alarm is actuated.